1. Field of the Invention
This invention relates generally to an apparatus and method for determining the moisture content of wood and paper products, and more specifically, to an apparatus and method utilizing a resistance or capacitance-controlled oscillator circuit integrally incorporating in the circuit the wood or paper product sample.
2. Description of the Prior Art
The measurement of the moisture content of wood and paper products is relevant to many aspects of the wood and paper industry as well as to users of wood and paper products. Certain physical characteristics of such products directly relate to moisture content of the product. Therefore, manufacturing processes involving these products must be adjusted in accordance with the moisture content of the products. While changes in physical characteristics of products in relation to changes in the moisture content of the products are generally well known, there previously has existed no convenient and reliable apparatus or method for quickly and non-destructively determining the product's moisture content in an assembly-line setting or during long-term storage.
Several methods and devices have been used in the past to measure the moisture content of wood and paper products. One such method is to remove a sample portion of the product, weigh the test sample, heat the sample in an oven to drive off the moisture, then re-weigh the dry sample. The change in weight of the sample divided by the weight of the "wet" sample before drying gives the moisture content of the product. This method requires a large amount of time and is not completely accurate primarily as a result of changes in the moisture content of the test sample during the time between removal from the product and drying.
Another method for testing the moisture content of a product is described in the Rosenau U.S. Pat. No. 4,259,633. Rosenau discloses a method of inserting metal pins into a product and passing a direct current voltage through the sample. Owing to the fact that the resistance of wood and paper products varies with the moisture content of the products, the amount of current conducted by the sample at a given voltage generally corresponds to a particular level of moisture. However, in the Rosenau method, electrolytic polarization effects create a reverse voltage which can introduce a large degree of error into the measurement. Direct current measurement techniques also have inherent problems with zero level drift and stray charges which become especially troublesome when trying to measure the relatively high resistances characteristically found in wood and paper products.
Still another method for measuring the moisture content of wood and paper products includes the subjection of the products to microwave or infrared radiation. This method determines the moisture content of the sample by measuring the absorbtive loss in the impinging radiation after passing through the product and calculating the amount of moisture which would cause such a degree of loss in radiation. Although this method may be accurate, it requires rather elaborate and sensitive equipment that is not easily transported or inexpensively operated and maintained. Also, access must be had to both sides of the product to be measured. Consequently, it could not be used where sources of moisture other than the product to be measured may be interposed in the measurement path. In addition, this method affects the moisture content of the product since the microwave radiation heats the product, thereby driving off moisture. The microwave radiation method is therefore unsuited for monitoring time dependent responses of products to the environment.
It is therefore an object of this invention to provide a moisture detector for wood or paper products which employs a resistance or capacitance controlled oscillator circuit integrally including in the circuit the wood or paper product in order to measure the moisture content of the product, thereby overcoming the problems posed in the prior art including electrolytic polarization and zero level direct current drift, as found in detectors using direct current voltage.
It is another object of the invention to provide a moisture detector for wood or paper products capable of measuring the moisture-dependent resistance or capacitance of such products in spans comprising more than five orders of magnitude and automatically compressing the data through use of a resistance or capacitance-controlled oscillator circuit having a response logarithmically related to the measured resistance or capacitance.
It is a further object of the invention to provide a moisture detector for wood or paper products utilizing probes designed to enhance the effective measured resistance or capacitance and further designed to reduce errors due to fringing electrical fields and relative placement of the probes on the wood or paper products.
It is another object of the invention to provide a moisture detector for wood or paper products which is easily transportable and easily installed in areas difficult to reach with other detecting apparatus.
It is a further object of the invention to provide a moisture detector for wood or paper products which enables very rapid and accurate determination of the moisture content of the products.
It is another object of the invention to provide a moisture detector for wood or paper products which includes a probe and a resistance or capacitance controlled oscillator circuit to monitor the moisture content of the product over a long period of time without excising or destroying a portion of the product for moisture assay and without materially affecting the moisture content of the product such as experienced with the moisture assay and microwave methods described with reference to the prior art.
It is yet another object of the invention to provide a moisture detector for wood or paper products which includes a probe permanently applied to the surface of the product for measuring the moisture-dependent resistance or capacitance of the product through incorporation of the probe in a resistance or capacitance controlled oscillator circuit, thereby making the wood or paper product an integral component of the circuit.
It is a further object of the invention to provide a moisture detector for corrugated wood or paper products which includes a plurality of paired electrodes inserted into flutes of the corrugated product, thereby measuring the moisture-dependent dielectric constant of the product using a capacitance controlled oscillator circuit integrally comprised of the corrugated product.